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SubjectRe: [PATCH 10/11] ptrace: move JOBCTL_TRAPPING wait to wait(2) and ptrace_check_attach()
On 05/12, Tejun Heo wrote:
>
> Hello, Oleg.
>
> On Wed, May 11, 2011 at 09:53:33PM +0200, Tejun Heo wrote:
> > > As for ptrace_check_attach(), it can simply do wait_event(), we
> > > only need to verify the caller is the tracer. No need to play with
> > > lock/unlock/retry.
> > >
> > > What do you think?
> >
> > Hmmm... interesting. Yeah, the state is visible only through wait(2)
> > and ptrace(2) and for wait(2) TRAPPING is as good as STOPPED/TRACED
> > and we can wait all we want in ptrace_check_attach(). I'll think more
> > about it but seems like a nice idea.
>
> Eh, problem. Please consider the following scenario.
>
> * A task is in TASK_STOPPED. current->exit_code contains zero.
>
> * Tracer seizes the task which triggers TRAPPING.
>
> * Tracer wait(2)s which sees TRAPPING but tracee->exit_code is still
> zero.

At first glance, this is easy to fix. do_signal_stop() can set
->exit_code unconditionally as it did before.

> I think retrying and ensuring that tracee is in the expected state
> after going through the regular trapping procedure is better way to
> handle this. Bypassing wait(2) seems smart and simple but it entails
> much more subtleties than simple stupid waiting/retrying.

Agreed. But this wait_event(!TRAPPING) logic has a lot of subtleties
too.

Oleg.



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